Photosensitive elements which can be used in image-reproduction processes are well-known in the graphics arts industry. Such elements are usually exposed to actinic radiation through an image-bearing transparency, such as a color separation transparency, to produce an image which is either a positive or negative with respect to the transparency used.
Such photosensitive elements are widely used in off-press color proofing to simulate the images produced by printing. In a surprint proof, all of the colored images are superimposed, by, for example, multiple exposure, lamination, or transfer, onto a single support. Unlike an overlay proof, the colored images cannot be separated and viewed individually.
Various processes for producing copies of images involving photopolymerization and thermal transfer techniques are known as disclosed in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; 3,574,049 and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed to a photographic transparency. The surface of the exposed layer is then pressed into contact with image receptive surface of a separate element, and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferable, unexposed, image areas of the composite, transfer to the image receptive element. If the element is not precolored, the tacky, unexposed image may now be selectively colored with a desired toner. All of these processes necessitate the use of specially treated final receptor sheets and are not applicable for obtaining a color proofing image on a paper stock.
If the element is precolored, flexibility in your choice of colors is limited because preparation of the precolored elements in all the desired colors is not economically feasible. Toning provides greater color flexibility but is associated with environmental concerns because the fine powders are difficult to handle. Also, use of toners results in images having significant relief so that it is difficult to obtain high resolution images when it is necessary to superimpose images to make multicolored images.
Precolored wash-off systems are known in the art. These include precolored diazo based imaging systems such as Van Beusekom, U.S. Pat. No. 3,671,236; Cederburg, U.S. Pat. No. 4,656,114; and Sachi, U.S. Pat. No. 4,666,817 which are developed in a mixture of water and 1-propanol; Krech, U.S. Pat. No. 4,260,673 which is developed in base; and Platzer, U.S. Pat. No. 4,751,166, Mino, U.S. Pat. No. 4,783,390 and Adolphson et al., U.S. Pat. No. 5,075,722 which are developed in water. These systems provide improved resolution. However, the elements are precolored and thus, color flexibility is limited because preparation of the precolored elements in all the desired colors is not economically feasible. An additional drawback for systems developed in aqueous base or organic solvents is that they can pose flammability, toxicity, corrosion and/or waste disposal concerns.
Thompson, U.S. Pat. No. 4,737,436 discloses a water based method for making color proof images on a single substrate using a photoresist coating containing a pigment blend. The process provides for the blending of pigments, followed by the combining of the pigment blend with a water soluble photoresist composition, coating a substrate with the photoresist pigment-containing blend, imagewise exposing and developing to produce an image on the substrate. A second image is then formed on the exposed and developed substrate by forming a second pigment blend, followed by the combining of the second pigment blend with a water soluble photoresist composition, coating the substrate having the first image with the photoresist second pigment-containing blend, imagewise exposing and developing to produce a second image on the substrate. This process has several disadvantages. First, combining the pigment blend with the photoresist coating can result in coating solution stability problems. Coating non-uniformity problems can result, with uniform coating becoming increasingly difficult as the size of the work increases. Second, since the pigment blend is mixed with the photoresist prior to coating, compatiblity of the pigment with the photoresist coating becomes an issue. This process is also limited in that it does not provide for the application of colorant to the approximate image areas only.
A need exists for a process that generates a high resolution, high quality color image on virtually any paper stock. Further, a need exists for a process that provides color flexibility without the environmental concerns associated with the previously mentioned proofing systems. A need also exists for a process that provides for application of the colorant to the approximate image areas only rather than the entire photosensitive layer.